Fixed my pdt11/150

Chris Zach cz at
Sun Apr 26 15:33:25 CDT 2020

So I've had a boat anchor pdt11/150 here for awhile. It's probably one 
of the weirdest pdp11s ever built: An 11/03 CPU ish, six serial ports, 
ish, and a pair of RX01 drives.


The trick is the system is very closed in: There are 4 boards inside 
with a lot of early microprocessors to do the IO instead of a real Q 
bus. The bottom board is a controller that is sort of like an RX01 but 
instead of using the DX: driver it uses a special PD: driver. The CPU 
connects to this with a 14 pin DIP ribbon cable, and on the back of the 
CPU module is a 64kb memory module and a serial module that has a 
console, printer, modem and three additional serial ports that are their 
own thing.

Problem with this one was that it would not come up. Serial tests seemed 
to fail using an error code of waiting for input which didn't make a lot 
of sense. So today I decided to pull the serial board and see if I could 
swap the UARTs.

I quickly figured out the problem: The serial board "connects" to the 
main board by two sets of bars with three screws each that hold the 
board to an interconnecting header that sends the signals. As soon as I 
loosened the screws I realized that the header isn't connecting to pins 
on either board, it literally presses against pads on the boards that 
complete the circuit. Nothing but pressure and springiness holds it 
together. No screw, pin and socket, anything.

With that I cleaned off the headers and wiped down the pads on the 
boards till they shined like the top of the chrystler building. I then 
reassembled and torqued the screws down evenly, finishing with the 
center screw first followed by the outside screws. It is to note that 
the hinges that hold the CPU and memory/serial boards to the body of 
this thing attach to the bottom of those screws so when you open it up 
you are flexing the assembly and probably stretching the screws a bit. 
Which results in bad contact...

Plugged it in and all is well.

RT-11SJ (S) V05.01C


SPCINV.SAV    10  21-Mar-1989    OTHELO.SAV    45  21-Mar-1989
SPCINV.DAT     1  21-Mar-1989    TODAY .SAV    20  22-Feb-1988
DECMAN.SAV    14  21-Mar-1989    SPACWR.SAV    13  21-Mar-1989
STRTRK.SAV    54  21-Mar-1989    SWAP  .SYS    26P 27-Jul-1984
RT11SJ.SYS    64P 19-Jun-1988    TT    .SYS     2P 19-Jun-1988
PD    .SYS     3P 19-Jun-1988    DX    .SYS     4P 21-Jan-2000
PIP   .SAV    30P 21-Jan-2000    DUP   .SAV    52P 21-Jan-2000
DIR   .SAV    20P 21-Jan-2000
  15 Files, 358 Blocks
  128 Free blocks

Another little DEC mystery solved. One odd thing about these: There are 
only four chip slots for the CPU and microcode, but one of the carriers 
has two dies on it so the system *does* have EIS and FIS instructions. 
Why not...


More information about the cctalk mailing list